The time-modulated color image intensifier has been described in U.S. patent application Ser. No. 07/622,268 entitled COLOR IMAGE INTENSIFIER DEVICE, filed Feb. 28, 1991 by R. Jett Field, the inventor herein, and assigned to ITT Defense Technology Corporation, the assignee herein, which said patent application disclosure is incorporated by reference herein. As can be seen from application Ser. No. 07/622,268, the time-modulated color image intensifier contains a standard color-blind image intensifier with a white output phosphor. Two moving filters are added to convert this to a color-sensitive device. The first filter is at the input, and filters the incoming light to make the device sensitive to only one color at a time. The second filter is at the output, and filters the white phosphor light to produce only one color at a time. The two filters would typically contain three elements each, such as red, green, and blue. The filters are moved synchronously so that when the first input element is in place and the device is sensitive only to the first input color, the first output element is also in place and the output light is tinted by the first output color. The filters are moved rapidly enough so that the eye perceives a full color image without flicker. The input and output elements can be matched for true color reproduction, or mismatched for false color.
In the simplest configuration, the two filters are color wheels rotating on the same axis. Unfortunately, this increases the size of the overall image intensifier system since each wheel must be approximately twice the diameter of the intensifier tube. Furthermore, in the case where the intensifier tube employs a 180.degree. image twister, the two wheels must be mounted on two different axles which further increases the size of the system.
In an alternate scheme shown in application Ser. No. 07/622,268, which is specially suited to compact systems, the filters can reciprocate on a linear track. In this case, the filter size may approximate the size of the intensifier tube, being only slightly larger than the image size. The filter elements are color stripes which may be narrower than the image size, so that different parts of the image are sensitive to different colors at any instant. Each of the three colors of elements should cover points on the image for equal times. In the simplest case, where each of the three elements has stripe width L, and the filter period is 3L, the filter could reciprocate distance 3L in a linear or "triangle wave" motion. Each image point is covered by the motion of three or four adjacent filter elements so that the exposure times of each of the three colors are equal. The overall length of the filter would be the image diameter+3L. If the intensifier tube employs a 180.degree. twister, then the filters must move out of phase with one another. (Precise out-of-phase reciprocation is easily implemented by mounting the reciprocating drive wheels 180.degree. apart on a common drive shaft).
The primary difficulty with the reciprocating scheme is in forcing the filters to move in a triangle wave motion. The simplest motion would be a sine wave as would result from the operation of a device like that shown in FIG. 1 of the present application, or a sinusoid wave as would result from the operation of a device like that shown in FIG. 2. FIG. 1 depicts a reciprocating filter bar 10 comprised of alternating color segments 12 of colors A, B and C which would actually be three primary colors, such as red, green and blue. The bar 10 is reciprocated on track 14 in direction x as depicted by double arrow 16 and passes in front of image intensifier input (and/or output) aperture 18. This reciprocating motion is accomplished via pin 20 riding in slot 22 of the bar 10. The pin 20 is rotated upon crank wheel 24.
FIG. 2 depicts a reciprocating arrangement like that shown in FIG. 1 but driven by connecting rod 26, connecting pin 20 and drive pin 28. In each of these cases, however, the three colors A, B and C would have unequal exposure times at each point on the image. For example, given sine wave motion with peak-to-peak motion range of 3L, and an image point which is covered by an element boundary at each extremum of this motion as in FIG. 3, such a point would be covered by the full width of three adjacent filter elements, but would be covered by the center element for a shorter time. As a result, images in this system would have uneven color balance in stripes with period 3L.
It is therefore an object of the present invention to provide a means for compensating for the non-uniform color exposure in a reciprocating-filter, time-modulated color image intensifier, such that more uniform color balance is achieved.